Hey, Robert :)
Seems to me you might have some doubts about either 1) your relationship with your wife or 2) the social impropriety of the young lady situation. It also sounds like you'd rather your MIL take her behind somewhere else so she will get off your wife's back.
Hmmm....a history re-enactor sounds wonderful! I know a few men who do this and I belong to a Yahoo group (18thcentury ladies or something like that) who are so historically-correct that they attempt to get the fabric that the women wore with their dresses during that time period. I don't participate but I love to read about what they do. I applaud that you are socially involved in a hobby that's important and enjoyable (I'm an amateur genealogist).
It sounds like to be you have an issue with the MIL being in your home. Criticism, in any form loud or soft, essentially hurts. As a man, sometimes you show it - sometimes you don't. I know my husband is like this. I've learned that if I want to keep the peace in our home, I have to believe that not everything is a personal attack against me or my own thoughts on family or social issues that I might have an opinion on.
His son lived with us 2x and during those times, I've never had my patience tried more. I'll never ask God to give me patience again, for sure, because those lessons are burned into my soul! God really knew what he was doing when he answered my prayers for more patience! Perhaps, your wife is going through a guilt of some sort. I felt guilty about my feelings towards my son because he was a constant stream of criticism and just downright mean sometimes. It hurt. I'm sure it tires your wife out to no end to know what she's faced with when she walks through the door each and every time she comes back from the small amount of freedom every day (work, errands - anything away from the house).
In my humble opinion (IMHO), I don't think your wife is ignoring you. I think she's tired of the homelife situation and doesn't know what to do about it. Women sometimes feel a certain obligation to their parents at the expense of their own personal desires. Have you thought about alternative living arrangements for the MIL? It could be that she really NEEDS social interaction with folks her own age that she can relate to. It really helped that my grandmother lived in an "older" neighborhood so she could walk up the street to talk to her old-lady-network and gossip about family and friends. Old women thrive on that sort of stuff :)
As for the young lady, you'll need to exercise some caution here. You, sir, are in a vulnerable position in life. You are lonely.
And loneliness can lead to trouble.
I would suggest less frequent associations with this young lady and start hanging out with men who share a common bond with you. Doesn't have to be men your age - as long as they share a knowledge and interest. I would also re-consider going to banquets and social functions without my wife. It not only causes gossip but, eventually, someone will feel sorry for you and introduce you to other women. The possibilities of hurting your wife is great at this point.
Your wife should re-consider her role in the home. SHE'S the woman of the house - not her mother. If she doesn't start putting her foot down (or in it), then she'll continue to feel guilty, lonely, frustrated, angry and totally and completely unsocial to everyone around her. But this is your wife's decision to make - you can't make it for her. Either she takes a stand for the sake of her marriage and her home or she can continue to be whipped by life and circumstances.
Ok, yeah, so I'm long-winded. What woman isn't? :)